The present invention relates to a brake control device in an air over hydraulic brake system mounted on a motor vehicle.
As a conventional technique, a brake control mechanism for controlling the brake fluid pressure of wheel cylinders has been known, in which an air over hydraulic cylinder including a built-in air piston on one side and a built-in hydraulic piston on the other side, and a pneumatic pressure fed from the air piston is changed to a fluid pressure, for controlling the fluid pressure in the wheel cylinders.
The provision of the modulator stated above with solenoid valves (hold and decay valves) mounted as one unit (a valve unit) is considered for the suction of air into, holding of the air within, and discharge of the air from, an air chamber of the modulator. There has been proposed in U.S. Ser. No. 07/694,341 a single valve unit, as an example, including the hold valve and the decay valve.
FIG. 8 shows the constitution of the valve unit mounted in such a modulator. Hereinafter there will be explained the major constitution of a valve unit 6. In the valve unit 6 a hold valve 4 as a first valve device and a decay valve 5 as a second valve device are arranged in parallel. A valve block 7 which comprises the hold valve 4 and the decay valve 5 and a solenoid block 8 which generates an electromagnetic force is securely fastened in one body by means of through bolts 19.
The hold valve 4 has a hold plunger 23 which is movable in an axial direction in the hollow section of a solenoid coil 20a. The bottom end of this hold plunger 23 is in contact with the top end of a valve pin 25 being pressed upwardly by a hold spring 24 inserted in the valve block 7. The bottom end of this valve pin 25 is expanded in a hold valve chamber 11 (a first valve chamber). At the bottom end face of the valve pin 25 is installed a valve rubber 26, forming a hold valve body 15. In the hold valve chamber 11 which is the first valve chamber, a valve receiving body 13 is provided under the valve pin 25. Provided in the interior of this valve receiving body 13 is an air suction passage 12 drilled in an axial direction for communication with an air inlet 10. Furthermore, there is formed an opening 14 in the top end face of the aforesaid air suction passage 12, opposite to the hold valve body 15. In the periphery of the valve receiving body 13 is fitted an O-ring 34 for preventing air leakage from the inlet 10 side to the hold valve chamber 11 side when the valve is closed.
At the bottom end of the valve receiving body 13 is installed a return valve 28 which permits only the flow of the air from the hold valve chamber 11 to the air inlet 10 side, being pressed upwardly by a return spring 30.
In the hold valve chamber 11 the chamber passage communicating with the air chamber of the modulator is open.
In this description the decay valve will not be explained.
It is conceivable that an accelerator slip control mechanism can be used correspondingly to such a modulator.
As a prior-art example, there has been disclosed a brake control device equipped with an acceleration slip control mechanism in Japanese Patent Publication No. Hei 1-54226. The brake control device disclosed in this patent is provided with a changeover valve which is located between the brake valve and brake pressure supply solenoid valve and the wheel brake solenoid valve.
This control device functions to selectively supply the higher of the pressures from either the brake valve or from the brake pressure supply solenoid valve, to the wheel brake solenoid valve. The control device controls the fluid pressure to be supplied to the brake wheel cylinders, in accordance with a command from a comparison electric circuit device, thereby performing antilock control and traction control.
According to the aforementioned prior-art brake control device, the air in the air chamber is discharged out at the brake valve through the air inlet 10 from the hold valve 4 side at the time of common release of the brakes. Therefore, there exists a great resistance in the line from the air inlet 10 to a brake valve, reducing an air discharge capacity and accordingly resulting in lowered responsivity at the time of release of the brakes.
In the brake control device which serves also as a traction control mechanism, the air from an air reservoir is supplied directly from the brake pressure supply solenoid valve to the wheel brake solenoid valve via the changeover valve; there is, therefore, a problem that the built-up rate of pressure application increases as compared with the ordinary braking effected by a brake pedal pressure applied by a driver, resulting in deteriorated brake controllability and vehicle body vibration. If one tries to adjust an increase or reduction of pressure by means of the comparison electric circuit and the wheel brake solenoid valve, the control method will become difficult to perform and component parts of the brake control device will become complicated and expensive.